The separation of a relatively low-density liquid, e.g. oil or an organic solvent, from a relatively high-density liquid such as water with which the low-density liquid is substantially immiscible is required for many purposes. For example, oil and water mixtures may arise in oil refineries or in the removal of oil from bodies of water and can be introduced into decanting tank or vessels in which the two liquids are permitted to separate by stratification. The relatively low-density liquid forms a layer above the relatively high-density liquid and must be removed.
Simple decanting techniques whereby the low-density liquid is permitted to flow over a weir at a fixed location on the separation vessel have not proved to be effective for many purposes especially because the level of the liquid may vary.
There has also been suggested a portable floating device for drawing pumpable impurities from the surface of a body of water. This device comprises an upper open surface and floats carrying the collecting vessel which comprises an upper receptacle portion and a lower receptacle portion, the two portions being so interconnected that they permit relative vertical movement. The lower receptacle portion contains one or two immersion pumps. The upper edge of the upper receptacle portion lies substantially at the upper surface of the water and the depth to which the upper edge can be immersed can be controlled by regulating the floats.
This conventional system has, necessarily, a complex and costly construction and is not always satisfactory in operation. For example, it cannot be used in large clarifier or settling tanks whose lateral walls are of rectangular cross section and have lengths of 4 to 6 meters or more.
Such large clarifier or separating vessels are used in the petroleum industry, e.g. at refineries, for the separation of large quantities of waste water which may contain petroleum products. Such products must be removed from the surface of the water in the vessel so that, for example, they may be worked up to recover the valuable petroleum components.